1. Field of the Invention
The present invention relates to an apparatus which dips a substrate, such as a semiconductor substrate and a liquid crystal glass substrate, into processing liquid to thereby perform surface processing. More particularly, the present invention relates to a substrate processing apparatus which stores processing liquid consisting of a plurality types of liquid in a processing bath, and heats the processing liquid to a temperature which is equal to or higher than a boiling temperature of at least one of the plurality of types of liquids consisting the processing liquid, and dips a substrate into the processing liquid.
2. Description of the Prior Art
During etching of a silicon nitride film which is formed at a surface of a semiconductor substrate using a phosphoric acid solution (H.sub.3 PO.sub.4) of a high temperature, for example, a change not only in the temperature of the phosphoric acid solution but also in the concentration of the phosphoric acid solution changes an etching rate, thereby resulting in an uneven processing result. Hence, it is necessary to maintain the temperature and the concentration of the phosphoric acid solution within a tolerable range.
However, as it is necessary to keep the phosphoric acid solution at a high temperature around 150.degree.-180.degree. C. during such an etching process, moisture in the solution easily evaporates and accordingly increases the concentration of the phosphoric acid solution, whereby the etching rate is changed and a processing result becomes uneven.
To deal with this, the following four types of methods have been principally used to appropriately add de-ionized water to the phosphoric acid solution during etching.
1. The method described in Japanese Utility Model Laid Open Official Gazette No. 2-98631, which requires to dispose a nozzle having a wall member which is perforated with a number of very fine through holes in a lower portion of a processing bath which is filled with a phosphoric acid solution, so as to supply de-ionized water to the phosphoric acid solution through the nozzle. PA1 2. The method described in Japanese Patent Publication Official Gazette No. 3-20895, which requires to supply de-ionized water to a circulation line which circulates a phosphoric acid solution by leading the phosphoric acid solution which is contained in a processing bath to adjust the temperature of the phosphoric acid, filtering the phosphoric acid solution, and returning the phosphoric acid solution into the processing bath once again. PA1 3. The method described in Japanese Utility Model Laid Open Official Gazette No. 59-84839, which requires to supply de-ionized water through a hole which is formed in a bottom portion of a processing bath. PA1 4. The method described in Japanese Patent Laid Open Official Gazette No. 6-69179, which requires to use a double bath consisting of an internal bath for etching and an external bath which surrounds the same, to always let a phosphoric acid solution flow over into the external bath from the internal bath, and to supply de-ionized water into the phosphoric acid solution stored in the external bath from above the external bath, so that the phosphoric acid solution containing the de-ionized water circulates into the internal bath.
By the way, among the de-ionized water supplying methods described above, the methods 1, 2 and 3 inject de-ionized water into the phosphoric acid solution and hence may cause abrupt boiling of the de-ionized water within the phosphoric acid solution upon injecting of the de-ionized water into the phosphoric acid solution.
Meanwhile, since the method 4 supplies de-ionized water to the surface of the phosphoric acid solution which is stored in the external bath, the de-ionized water stays at the surface of the phosphoric acid solution rather than diffusing into the phosphoric acid solution. Further, as the de-ionized water evaporates from the surface of the phosphoric acid solution, the temperature of the phosphoric acid solution remarkably decreases due to heat of vaporization.